Data communications by wireless LAN such as IEEE802.11 are now widely used. The wireless LAN is used, for instance, in personal computers (PCs), PC peripherals such as printers and hard disk drives, electronic equipments such as FAXs, standard televisions (SDTVs), high-definition televisions (HDTVs) and mobile phones, signal-transmitting means in place of wired communications in automobiles and aircrafts, etc.
A high-frequency circuit used in a multiband communications apparatus for such wireless LAN comprises one antenna capable of conducting transmission and reception in two communications systems having different communication frequency bands (for example, IEEE802.11a, and IEEE802.11b and/or IEEE802.11g), and a high-frequency switch for switching connection to a transmitting circuit and a receiving circuit, thereby switching the transmitting circuit and the receiving circuit for two communications systems. As wireless apparatuses have been miniaturized and provided with higher functions, there is increasingly stronger demand to integration and miniaturization on high-frequency devices with high-frequency circuits.
In such high-frequency circuits, bandpass filters selectively passing signals in predetermined bands are important circuits. The bandpass filter is arranged between a front end of an antenna circuit and transmission and receiving circuits, etc. to remove unnecessary waves outside the passband. The bandpass filter is required to have not only steep filtering characteristics near the passband, but also high attenuation in bands such as harmonic bands outside the passband. In addition, it should be miniaturized and have higher functions.
WO 2008/066198 discloses a small, laminate-type bandpass filter comprising three resonators, resonance lines for the resonators being constituted by connecting electrodes formed on pluralities of layers in parallel. The bandpass filter with such structure has low impedance, low insertion loss and excellent attenuation characteristics. However, this bandpass filter suffers electromagnetic coupling between transmission lines, because the resonance lines are constituted by pluralities of transmission lines. When intervals between the resonance lines are narrowed according to the demand of miniaturization, there appears strong electromagnetic coupling, resulting in increased insertion loss. Thus, the bandpass filter of WO 2008/066198 cannot be miniaturized with low insertion loss. Also, because resonance lines are formed on pluralities of dielectric layers, filtering characteristics vary with the lateral displacement of lamination putting the resonance lines out of alignment in a lamination direction.